1. Field of the Invention
The present invention relates to a fixing device which fixes an unfixed toner image to a recording material by a hot pressing method, and an image forming apparatus having the fixing device.
2. Description of the Related Art
A fixing device of a heating roller fixing system is generally used in an image forming apparatus of an electrophotographic system, such as copiers, printers and multi-function peripherals. The fixing device is constituted of a fixing roller and a pressure roller, which are in pressure-contact with each other, and surfaces of both the fixing roller and the pressure roller, or surface of either of those rollers are heated to a predetermined temperature (fixing temperature) with a heat source (for example, halogen lamp) provided in the rollers.
A recording paper having an unfixed toner image transferred thereto is passed through a pressure-contact portion (fixing nip portion) between two rollers, thereby a toner image is fixed to the surface of a recording paper by heat and pressure.
An elastic roller provided with an elastic layer comprising a silicone rubber or the like is generally used as a fixing roller in a fixing device used in a color printer.
When the fixing roller is provided with an elastic layer, the surface of the fixing roller elastically deforms in response to irregularity of an unfixed toner image on a recording paper, and comes into contact with a recording paper so as to cover a face having a toner image formed thereon. This permits to well carry out heat fixation to a color printing in which an amount of a toner constituting an unfixed toner image is large as compared with a monochrome printing.
Furthermore, a strain release effect of an elastic layer in a fixing nip portion can improve releasability to a toner even in a color printing which easily induces offset as compared with a monochrome printing.
Moreover, the fixing nip portion has an inverted nip shape, that is, a state where a portion of a fixing roller coming into contact with a pressure roller gets slightly dented. As a result, release performance of a recording paper is improved, a recording paper is easily released from the surface of a fixing roller without using a release member such as release nail, and an image defect caused by a release member can be prevented.
The elastic layer has the above-described good fixing characteristics, but on the other hand, has low thermal conductivity. Therefore, in the case where a heat source is provided in a fixing roller equipped with an elastic layer, heat transfer efficiency to the surface of a fixing roller is decreased. As a result, a warm-up period is prolonged. Furthermore, in the case where conveying speed of paper is increased for high-speed printing, there is a problem that surface temperature of a fixing roller does not follow the speed.
An external heat fixing method in which an external heating section is brought into contact with the surface of a fixing roller and the fixing roller is heated externally (surface side) is known as means for solving the problem arising from low thermal conductivity.
A roller method of coming into contact with a heating roller having a heat source therein with the surface of a fixing roller; a belt method of heating an endless belt member and bringing the belt member into contact with the surface of a fixing roller; and the like are proposed as the external heating section. The belt method can ensure a wide heating nip width with smaller heat capacity as compared with the roller method. Therefore, amount of heat that can be supplied to the surface of a fixing roller is large, and follow-up properties of the surface temperature of a fixing roller in high-speed printing are excellent.
On the other hand, the belt method has the following problem. When rotation of a fixing roller is stopped after completion of fixing operations, a heating member which heats the belt member comes into contact with the fixing roller with the belt member interposed therebetween. As a result, the contact portion is locally heated, thereby accelerating deterioration of an elastic layer. This local overheating is hereinafter referred to as “overshoot”.
In other words, in the belt method, heat supply ability to a fixing roller is high, and a plurality of members such as the belt member and belt supporting rollers are interposed between a heat source and the surface of a fixing roller. This increases a temperature gradient between the heat source and the surface of the belt member. As a result, when rotation of the fixing roller is stopped and heat transfer to the fixing roller is instantaneously blocked, temperature of the belt supporting rollers and the belt member is increased due to the temperature gradient and a fixing roller portion with which a belt winding roller comes into contact through the belt member is locally overheated. In particular, because heat capacities of a belt winding roller, an endless belt and the like are small, the overshoot due to temperature gradient is markedly developed.
Further, generation of the overshoot induces the problem that temperature irregularity is generated on the surface of a fixing roller, resulting in occurrence of gloss irregularity on an image subsequently fixation-treated.
Furthermore, in a system requiring a change of surface temperature of the fixing roller according to image data to be printed, in the case of decreasing surface temperature of a fixing roller during fixing operations, a heat source disposed inside the fixing roller and a heat source disposed inside the belt supporting roller are turned off, and the fixing roller is rotated in this state, thereby decreasing temperature on the surface of the fixing roller.
However, temperature of the belt supporting roller is set to a temperature higher than a fixation temperature. This induces the problem that much time is required to decrease the surface temperature of the fixing roller.
These problems arise from the state where an external heating section is always in contact with a fixing roller. Therefore, when the external heating section is constituted such that the external heating section can be separated from and brought into contact with the fixing roller, those problems can be solved.
In the case where separation/contact operation of the fixing roller is allowed, the heating member has a temperature higher than the temperature on the surface of the fixing roller even when heating is stopped. Therefore, there is a problem that when the belt member is completely separated from the fixing roller, heat transferred to a portion which comes into contact with the heating member concentrates in the vicinity of the contact portion in the belt member, and this accelerates deterioration of the belt member. In the constitution that even the belt member is completely separated, movement distance necessary for separation is large. This induces problems that a wide space is required for the movement of separation and contact, and a space-saving constitution is not achieved.
Japanese Unexamined Patent Publication JP-A 2007-248724 discloses a fixing device, characterized in that a belt member and a fixing member come into contact with each other in a state where the heating member is separated from the fixing member, and the belt member rotates by following rotation of the fixing member. This fixing device is hard to deteriorate the belt member and can achieve a space-saving constitution, an electric power saving and a quick separation/contact operation.
However, in the case where the heating member in a separated state comes into contact with the fixing member by a quick separation/contact operation, impact force is transferred to an endless belt, a fixing roller and a heater lamp mounted in a heating member with a shock by the contact, resulting in a breakage of an endless belt or a fixing roller, and a positional deviation or a damage of a heater lamp.
When the heating member is moved, in the case where the heating member is not in pressure-contact with a fixing member under an appropriate pressure, a driven rotation of the belt member is not smoothly carried out, causing a poor rotation. This induces a lack in heat supply to the fixing member and a deterioration of the belt member.